Many devices fabricated from silicon and other semiconductors can not be constructed from two dimensional structures, but must be truly three-dimensional. Some examples of these three-dimensional devices include micromechanical sensors and actuators, electrostatically driven micro-motors, intricate micro-tubing and valve systems. Other three-dimensional structures are optical gratings and lenses, isolation trenches, mesas and via holes.
Three-dimensional structures formed from silicon substrates having p and n-type silicon regions are generally achieved by lithography and anisotropic etching processes. The structures which can be made by these processes are geometrically limited due to the finite number of &lt;111&gt; surfaces and the availability of wafers with different orientations. Additionally, in order to achieve the necessary etch stops in which etching of a desired silicon substrate terminates, the silicon substrate is generally highly doped. This causes mechanical stresses in the material which can warp the resulting structure leading to degradation of the micromechanical response of the device.
Accordingly, there is a need of an improved method for micromachining three-dimensional structures from substrates having both n and p-type silicon regions in which n-type and p-type silicon regions can be selectively etched without warping the resulting structure.